My little 400 was really wanted to spread her oats at Norwalk but a fuel problem had me fighting a bog all weekend. I changed to a stock 68 intake and a 1/2 inch spacer and i really felt a top end charge. best mph was 98 with RPM all weekend i was around 101.68.Cleaned fuel filter & still bog ( almost shuts off on starting line) then runs great!! maybe some depri,s in carb.?? I don,t know.